One thing about BHO method is time, the longer BHO and plant material are together the more icky stuff is extracted, like Chlorophyll (tastes bad!)!
In my humble experience, if the BHO takes longer than 1 min. from top to bottom it starts picking up unnecessary oils thus the green to black color.
Also, the colder the better! extracting at 40F is better than 80F, humidy @60% + , use a large glass plate like found in microwave ovens W/ a heating pad under them (Helps evap butane on plate quicker).
I'm not advocating making hash from males, just saying it works if there's nothin' else....................